The first goal of this project is to continue to lower the exposure to arsenic (As) of a cohort of 24,000 men and women recruited under the Health Effects of Arsenic Longitudinal Study (HEALS). This will be achieved by testing the numerous household wells that continue to be installed within the study area of Araihazar, using both laboratory measurements and field kits with support from Core C. In addition, the 110 deep community wells installed in some of the most affected portions of the study area will continue to be monitored. Beyond the ethical motivation, lowering exposure will help define exposure estimates of the studypopulation and, therefore, the dose-response relationships for various end-points studied under Projects 1, 2, and 3. The second goal of the project is to improve our understanding of the processes that threaten the quality of groundwater in aquifers that are presently low in As in Bangladesh by conducting field investigations. To determine the vulnerability of shallow (<30 m) aquifers that are tapped by most household wells, a ~1 km2 open field area located between a high-As and a low-As village of Araihazar will be densely instrumented and monitored for an entire year. Four ponds will then be dug and a nearby area raised with the fill to simulate the rural development of the region. The impact of this perturbation on the local hydrology and biogeochemistry (including As) of the perturbed areas, as well as two unperturbed controls, will be monitored for three years. To determine the vulnerability of deeper (>100 m) low-As aquifers, the origin of the failure of a handful of community wells in Araihazar will be investigated using a combination of geophysical, hydrological, and biogeochemical approaches supported under Project 4 and Cores C and D. The potential for downward incursion of shallow high-As groundwater on a broader scale due to intensive deep pumping below the city of Dhaka will be evaluated and modeled. These investigations will be carried out in the country where the health impact of As release to groundwater is by far the largest worldwide.